1. Field of the Invention
The present invention relates to a process for dissolving used catalyst based on a carrier containing alumina as the principal component. More specifically, the present invention relates to a process for dissolving used catalyst which is designed for recovering valuables from catalysts which were previously used in refining petroleum and then discharged as wastes.
2. Prior Art
From the viewpoint of effective use of resources, numerous attempts for recovering valuables from wastes have been made to present. Among the wastes, those which are recovered as valuables are generally used as secondary materials, and those out of interest from the recovery cycle are treated as either non-industrial wastes or industrial wastes; i.e., they, for example, are subjected to incineration or are used in reclamation. The determination of whether a particular waste should be recovered or not depends on the so-called economical and social environments, but it largely depends also on the technological level achieved at that time.
Considering the case of used catalysts which are discharged in large amounts from a hydrodesulfurization process in refining oil, for instance, these used catalysts contain alumina, the main component of the catalysts; molybdenum, nickel, cobalt, and other active metal components; heavy metal components such as vanadium and sulfur components derived from the raw material treated by the catalyst; and the decomposition product derived from oil, such as carbon, which is produced as a result of hydrodesulfurization reaction. It has been demanded for a long time, therefore, to effectively recover the valuable metal components from the used catalysts.
At present, however, only vanadium and molybdenum are recovered from the used catalysts. Only a part of the components, e.g., alumina and valuable metals such as nickel and cobalt, is used as cement materials at best, and the rest is treated as waste and mainly used in reclamation.
The insufficient recovery of the used catalysts is a consequence of the fact that these used catalysts contain carbon, oil components, sulfur components, and the like in addition to the valuable metals. Moreover, the oil components are sometimes present in a considerably large amount. These carbon, oil, and sulfur components must be removed, for example by oxidation roasting, before recovering the valuable metals and the like. However, when subjected to oxidation roasting, alumina reacts with metals such as nickel and cobalt to form a stable metal aluminate (complex oxide) having a spinal structure. Otherwise, alumina itself undergoes phase transition to acquire a stable .alpha.-Al.sub.2 O.sub.3 (corundum) structure. Because the complex oxide and/or corundum has extremely poor reactivity with acids and alkalis, it is found substantially impossible to recover metals such as nickel and cobalt by using any dissolution process using an acid or an alkali.
Other methods for recovering nickel or cobalt include reduction dissolution process. This process is believed to dissolve 70% or more of nickel or cobalt. When this process is used for recovering nickel and cobalt, however, about 50% of the alumina carrier is dissolved at the same time. This considerably makes the process uneconomical, because a complicated post treatment is required to separate and recover each of the metal components. It has been also attempted to completely dissolve alumina to facilitate the recovery of the other metal components. Such proposed attempts include using a mixed acid or carrying out the dissolution under a high temperature and high pressure using an autoclave. These proposed methods are capable to dissolve about 90% of alumina, but yet insufficient for the achievement of complete dissolution.